The present invention relates to a method of driving a glass tempering system, the system includes:
a loading section, PA1 a heating furnace including a thermally insulated chamber with heating resistances therein, PA1 a quench and cooling section, including cooling equipment fitted with air blowers, PA1 an unloading section, PA1 conveyors in each section and furnace, comprised of horizontal rollers arranged transverse to the glass conveying direction, PA1 drive mechanisms for the conveyors, PA1 clutch mechanisms for driving the separate conveyors in unison or separately. PA1 1. The wearing and breaking hazard of the quench and cooling section rollers is reduced. PA1 2. The design construction and system control are simplified. PA1 3. Thermal equilibrium of the furnace is maintained with all glass thicknesses. PA1 4. It is ensured that most of the glass sheets too susceptible to breakage in transportation, installation or use will be removed in advance.
In the method, the furnace conveyor is oscillated back and forth while heating the glass sheets to their tempering temperature and the quench and cooling section conveyor is oscillated back and forth at least while effecting the quenching of glass sheets.
German Patent specification No. 704 219 discloses a glass tempering system having a first index cycle during which the conveyors are oscillated for heating a glass sheet load in the furnace while a glass sheet load in the quench and cooling section is quenched and cooled while oscillating, and a second index cycle during which both conveyors perform a single long conveying stroke for carrying the glass sheet load from the furnace to the cooling section. If the mutually coupled furnace and cooling section conveyors are driven at the same speed, no effective length difference will be obtained between furnace and cooling section although this would be desirable to provide the furnace with a sufficient effective length and, on the other hand and regardless of that length, in order to make the cooling section as short and inexpensive as possible.
A long continuous-action furnace, connected with a short, oscillating conveyor equipped quench and cooling station, has been disclosed in U.S. Pat. No. 2,140,282. However, with relatively small glass production series and with the dimensions of glass sheets to be tempered often changing, it is desirable to drive also the furnace in an oscillating fashion since, in this way, the furnace can be more readily and quickly adjusted to changes.
Great Britian Patent Specification No. 1 527 782 discloses a solution, wherein both the furnace and quench and cooling section conveyors operate in an oscillating fashion connected to each other, but the quench and cooling section is shorter than the furnace. A necessary difference in speed between the conveyors in the furnace and the quench and cooling section is achieved by a reduction gear.
Another related disclosure is contained in U.S. Pat. No. 3,994,711, wherein the conveyors of the furnace and quench and cooling station are fitted with their own separate drive mechanisms for driving the conveyors independently of each other, i.e., with separate motors. Since this patent suggests that the length of the oscillations within the furnace is greater than the length of the sheet glass load to be heated (i.e., the furnace is at least twice as long as the maximum glass load length), the economically acceptable length of the quench unit is only a fraction of the furnace. Accordingly, the use of the independent conveyor drives made it possible to select the period, stroke length and/or speed of one conveyor for a given purpose regardless of the period, stroke length and/or speed of the other, i.e. to be equal or unequal depending on the situation. The long length of the furnace relative to the quench unit in practice substantially requires separate motors, i.e., independent drives, in order to maintain the speed of movement during quenching at a sufficient level while maintaining the glass sheet within the quench and permitting full length movement of another glass load in the furnace.
A corresponding solution has been proposed as early as 1930 in German Patent specification No. 511 244 which discloses that the conveyors of a glass annealing furnace be driven independently of each other, e.g., with separate motors, for driving adjacent, oscillating pre-cooling and cooling section conveyors at different speeds. This independent driving by two separate conveyors is due to the different requirements in the sections. However, this German specification also teaches driving the section conveyors at the same speed to effect the transfer of glass from one section to the other. Another patent disclosing oscillation of workpieces in a heating chamber and subsequently transferring the workpieces to an independently operated quenching chamber is U.S. Pat. No. 3,447,788.
Thus, it has appeared that the conveyors require separate drives independent of each other in order to provide a solution to the problem of giving the furnace and quench conveyors different oscillation lengths, necessitated by the relative lengths of the sections for economical operation and by glass sheet loads of unequal lengths.
However, it has been possible to design a suitable system even without the independent and separate conveyor drives. Applicant's German Patent specification No. 3 035 591 and corresponding Great Britain Patent Specification No. 2 059 941 describe a system wherein the conveyors are mechanically linked together and driven dependently on each other by a single drive motor. Graduation from independent separate drive to a single-motor drive was possible by basing the operation of a system on a new principle of controlling the movements. According to that principle, accurate information is acquired about the length of each glass sheet load and, while continuously observing the position of a carried glass sheet, a microprocessor is used to precalculate ratings for the movements of a glass sheet load in the furnace and in the quench and cooling section. With this operating principle, the length of the furnace can be shortened while retaining proper heating of the glass sheet. For example, the gear ratio for mechanically connecting the separate conveyors to a single drive mechanism may be calculated. Compared with a dual-motor drive disclosed in U.S. Pat. No. 3,994,711, this solution made it possible to simplify the system design substantially and to cut down the costs of a drive assembly since two independently driven but nevertheless mutually synchronized DC-motors with their control systems (as in U.S. Pat. No. 3,994,711) is approximately five times more expensive than a single AC-motor with its hydraulic variator for reversing the unidirectional rotation of a motor into alternating rotation at a desired speed and stroke as in Applicant's British and German patent specifications.
The present invention also utilizes a single drive motor, allowing the use of a simple and inexpensive hydraulic variator and its associated control system, since there is no conveyor speed synchronization problem when moving glasses from the furnace to the quench and cooling section. A normal hydraulic variator cannot be used in parallel with another similar hydraulic variator, since even a slight difference in the speed of rotation shifts the entire load on the hydraulic variator running at the higher speed of rotation. On the other hand, since a tempering process does not require highly accurate speed control (a 5% speed setting is quite sufficient), a special arrangement for coupling two hydraulic variators together would be unreasonably expensive due to doubled equipment and the need of particularly accurate control. In addition, such systems are complicated and susceptible to faults and thus hazardous in terms of a tempering process since, if a load of glass remains in the furnace because of some malfunction till overheating, such load may damage, for example, highly valuable ceramic rollers in the furnace. The present invention differs substantially from the above-described prior art in the sense that during the quench cycle, wherein a glass sheet is oscillated in the quench and cooling section at a sufficient rate of speed for quenching in order to prevent the sagging of a still soft glass sheet between the rollers or to prevent the formation of roller marks on a soft glass surface, the furnace conveyor is oscillated unloaded, i.e., without a glass sheet load at the same speed and stroke length as the conveyor in the quench and cooling section. Not until the quenching is completed, i.e., after the temperature in the center of the glass sheets is lower than the "strain point" of the particular raw glass material, is the movement in the quench and cooling section conveyor altered by activation of appropriate clutches so that a fresh load of glass may be passed into the furnace to be heated. While this glass sheet load is oscillated in the furnace and as soon as a glass sheet load in the quench and cooling section has cooled down to a suitable handling temperature, the quench and cooling section conveyor may be mechanically coupled with the unloading section conveyor and the glass sheet load carried from the quench and cooling section to the unloading section. The oscillation of a glass sheet load in the furnace is continued for a necessary period of time with the quench and cooling section empty, i.e., without a glass sheet load to be quenched or cooled. Alternatively, the glass in the quench and cooling section can be retained therein until the glass sheet load in the furnace is properly heated. Thereafter, the glass sheet load is transferred from the quench and cooling section to the unloading section simultaneously with the transfer of the glass sheet load from the furnace to the quench and cooling section. In a preferred embodiment of the invention, the coupling of the quench and cooling section conveyor with the furnace conveyor is changed, after a proper period of time for quenching has expired, from the 1:1 gear ratio to a reduction gear having a gear ratio so high that, at the maximum rate of oscillation of the furnace conveyor, the maximum travelling speed of a glass sheet in the quench and cooling section will be less than or equal to 2.5 cm/sec. This rate of speed is so slow that its use during quenching is not feasible. The purpose of this slow movement is explicitly to produce a suitably irregular cooling blast to the surface of an already quenched glass sheet to be cooled in order to produce strains in a glass sheet. Similar strains may be introduced in the glass sheets by providing an arrangement for permitting relatively slow, short movements of the cooling air nozzles. The significance of these strains will be explained later. The preferred embodiment of the present invention is especially useful when tempering a highly fragile glass sheet with holes near the edges and/or with the edges provided with recesses or the like irregularities.
In an alternative embodiment, after a suitable length of time for quenching has expired, the glass in the quench and cooling section is stopped completely to produce strains in the already quenched glass sheet from the blowing of air from the nozzles against the surface of the stopped glass sheet.
The embodiment in which the glass sheet load is moved very slowly in the quench and cooling section has certain operational advantages over the embodiment in which the glass sheet load is stopped completely after quenching. The loss of glass due to breakage in the quench and cooling section is slightly reduced since the glass sheets are moving slightly relative to the air nozzles. If the conveyor and the glass sheet load are maintained totally stationary, too great a strain may be introduced in the glass sheet causing breakage of a glass sheet which is actually properly tempered. Glass sheets having holes, notches, rough finishes and the like are particularly susceptible to breakage for such great strains even when the glass is otherwise properly tempered.
Moreover, the very slow speed movement of the quench and cooling section ensures that the roller themselves are not exposed to one-sided cooling from below or heating from the glass sheet load which one-sided heat effect may cause slight bending or curving of the rollers. This deformation of the rollers causes fluttering of the glass sheets subsequently transferred to the quench and cooling section. The fluttering increases the risk of breakage of the glass and may cause a reduction in the optical quality of the glass.
The method and system for embodying the present invention offer, e.g., the following important advantages:
The following description will deal with the reasons which lead the invention to the above advantages.
It has been found in practice that, when tempering glass, e.g., float glass, the temper or its uniformity will no longer change when temperature of the glass center on cooling passes the temperature corresponding to the "strain point", at this point the glass viscosity is according to definition 10.sup.14.5 poise.
The "strain point" of today's most commonly used float glass is typically slightly over 500.degree. C. It is a general fact that all presently manufactured flat glasses have a "strain point" higher than 500.degree. C. Therefore, in the tempering process if the temperature of glass center is lower than 500.degree. C., the quenching step has certainly been effected. At lower temperatures, the question is merely cooling the glass to a suitable handling temperature.
The quench times of glasses tempered to the normal degree of temper are as follows (the center temperature receding below 500.degree. C.):
______________________________________ Thickness Time ______________________________________ 3 mm circa 3.5 sec 4 mm circa 6.0 sec 5 mm circa 9.0 sec 6 mm circa 14.0 sec 12 mm circa 60.0 sec ______________________________________
When tempering glass, the uniformity of quenching at various points of glass is highly important since irregular quenching leads easily to extra losses, decreased strength of the final product and optical distortions (for example, bluish spots if polarized light is reflected from the glass). On the other hand, in the cooling cycle it has been found to be important to have sufficient irregularity of cooling to result in breakage of some of the faulty glasses since the warranty costs of glasses shattered during transportation, installation and use are relatively speaking extremely high when compared with the removal of faulty glasses in the production stage.
Against this background it can be appreciated, as described in more detail hereinafter, that one significant advantage gained by the present invention is the decrease of wearing and damage to the coating of the quench and cooling section rollers.
The coating of the quench section rollers generally comprises spirally-wound heat-resistant string, fiber-glass stocking pulled over the rollers or a plurality of rings threaded on the rollers (See, e.g., U.S. Pat. No. 4,421,482).
A general problem with the quench section rollers has been that their coating is readily worn down and damaged because of the sharp glass edges. Especially in a situation where the glass to be tempered shatters in the quench and cooling section, the roller coating is easily damaged. Breakage of glass in the quench and cooling section is statistically very common. Depending on the types of glasses to be manufactured, the thermomechanical properties of a tempering plant and the control values set by the operator, the breakage percentage varies within the range of 0.2-10%.
Shattering of glass occurs typically as follows:
In the quench cycle itself, it is generally impossible to notice any changes in glasses. Immediately after the quench cycle, when the inner parts of glass have passed the "strain point", the glass first breaks into larger fragments and later, as the glass cooling proceeds, these larger fragments keep "exploding" into smaller and smaller bits. The larger fragments of glass encountered at the start of the shattering process are the most inconvenient in terms of the quench and cooling section rollers, since the size of these fragments is often such that they drop between the rollers and remain upright on top of or between cooling nozzles, such that the sharp glass edges cut the coatings of the quench and cooling section rollers.
By substantially slowing or even stopping rotation of the rollers immediately after the quenching cycle in accordance with the present invention and by maintaining the slow movement or stoppage of the rollers during the stage of large glass fragments, any fragments from glasses that have "exploded" cannot damage the rollers but, instead, tend to fall down harmlessly between blast nozzle distributing ducts to the bottom of the quench and cooling section.
The fall of glass bits can be further facilitated by drawing the cooling blast nozzles away just before coupling the cooling section conveyor with the unloading station conveyor and removing intact glasses onto the unloading table. This movement of the blast nozzles provides more space between the nozzles and rollers, permitting the glass crumbs to fall down more freely.
The high gear ratio and the slow movement achieved thereby according to the preferred embodiment of the present invention is preferably utilized with all glasses including, in particular when manufacturing glasses that are highly fragile during the manufacturing process. These fragile glasses include glasses with holes near the glass edge or small-radius recesses at the glass edge. The purpose of the slow movement is just to slightly relocate the blowing during the cooling, so that, e.g., the neck (a very delicate spot) between the hole and the glass edge would not be continuously exposed to the center of a blast which could cause the breakage of glass even if such glass in terms of statistical probability were acceptably solid for external loads. However, this movement during cooling is so slow that, if used in quenching, the result would be irregularly tempered and hence unacceptable glass.
If the quench and cooling section conveyor is adapted to run through a high-ratio gear during the cooling cycle, the result achieved in terms of roller wear is not quite as favorable as that accomplished by stopping the conveyor completely but, however, a major improvement over the prior art is achieved since the travelling speed of the quench and cooling section conveyor rollers is less than 1/10 the speed of the furnace conveyor rollers. Moreover, substantial uneven cooling of the quench and cooling section rollers is avoided.
Another significant advantage gained by the invention is that, while it is possible to use just one drive motor, loading into the furnace can be carried out at a time different from unloading from the furnace into the quench and cooling section. Thus, it is not necessary to run an empty working cycle even if the loading on the loading section is not finished by the time the heating period in the furnace has run out.
If the high-gear ratio embodiment is utilized, loading of the furnace can only be effected when the glass in the quench section has indexed towards the furnace a distance A=K.sub.V L where: K.sub.V =transmission ratio of gear, e.g., 1:10 (preferably around 1:15); and L=length of loading movement from the loading table to the furnace end which faces towards the quench and cooling section.
There are no other limitations to the commencement of furnace loading and the calculation set out in the Applicant's prior
Great Britain Pat. No. 2 059 941 for selection of a transmission ratio is not needed at all.
A third important advantage gained is that the loading of a furnace is more uniform with all glass thicknesses. The reason for this is that the furnace conveyor is oscillated unloaded during quenching. The tempering plant furnaces are typically very massive, e.g., ceramic rollers retain in themselves a lot of thermal energy. Therefore, changing of the temperature in a furnace is a tedious procedure and even if the temperature could be changed quickly in the air space of a furnace, the roller temperature will change very slowly.
It has been found in practice that maintaining the furnace temperature constant at all glass thicknesses is highly important since oscillating tempering plants are typically employed in short series production, wherein the thickness of the glasses to be tempered varies several times a day.
On the other hand, the thermal equilibrium of a furnace is very sensitive to overloading. In particular, the temperature of the furnace rollers drops rapidly since, these rollers serve in practice as rotating heat exchangers between the lower section heating elements and the glass. The more heat that the roller must transfer per unit time, the greater their temperature drop with respect to the lower section temperature.
The temperature drop of the rollers, depending on the loading thereof, is a common problem to all roller-equipped horizontal tempering plants. For example, U.S. Pat. No. 3,994,711 suggests that the furnace of an oscillating tempering plant be twice as long as the longest possible glass (glass loading) such that the rollers have regular intermediate periods without a glass load for equalizing their temperature both from above and below. A problem in this type of solution is the high price due to the length of a furnace and the fact that the rollers are without a load of glass for varying times, i.e., the rollers in the center of a furnace are a lot colder than those at the ends of a furnace where the glass arrives seldom and stays a shorter period of time compared to the center rollers.
It has been found in practice that, in terms of the present requirements, the roller temperature is satisfactorily controllable if the heating time of glasses to be heated in a furnace to a tempering temperature is longer than 40 sec per mm of thickness. Thus, the heating time of, e.g., 6 mm glass should be at least 240 seconds. When a furnace is made substantially shorter, there is always a glass on some of the rollers aside from a simultaneous loading and unloading cycle, during which the rollers will be without a load of glass for a few seconds. Therefore, it is desirable to provide an arrangement for permitting the furnace to re-establish thermal equilibrium.
However, the heating time of glasses in a furnace is not directly proportional to the glass thickness since thicker glasses absorb more of the radiated power of a furnace. The real rate of heating to a tempering temperature typically follows the times set out in FIG. 3, assuming that the furnace temperature stays constant. As a matter of fact, if a furnace is set at the optimum temperature for 3 mm glass, the furnace will be overloaded by 20% on 12 mm glasses with a result that the optical quality of thick glasses suffers and the breakage of glasses during the tempering process increases dramatically.
As effected by the method according to the present invention, the heating rate of glasses is set out in FIG. 4. The furnace temperature has been increased in a manner that also 3 mm glass can be heated in optimum time. And with thicker glasses, use is made of the unloaded period of furnace which, with increasing glass thickness, grows longer in a manner that the average optimum load is achieved (compare the increase of quench time with the increase of glass thickness).
The unloaded period of the furnace is very beneficial since during this period the rollers receive compensating heat also from the top chamber of the furnace to further improve the thermal equilibrium of the rollers. With the thickest glasses in particular, the significance of thermal equilibrium has typically been most important since thick glasses are often also large in area, thus adding rapidly to the susceptibility to breakage and to the problems of optical quality.